To protect the contents for distribution, cold storage, treatment, such as heat treatment for sterilization, the packaging material used for packaging food and drink is required to have various functions, for example, high transparency for ensuring outside view of the contents, etc. in addition to mechanical properties, such as strength, resistance to cracking, and heat resistance. Recently, oxygen barrier properties for preventing oxygen from penetrating into the package to protect packaged foods from oxidation, carbon dioxide barrier properties, and barrier properties for preventing fragrance from escaping are also required.
Sheet and film made of polyolefin such as polyethylene and polypropylene, polyester such as polyethylene terephthalate (PET), or aliphatic polyamide such as nylon 6 have been widely used as the packaging material because of their easiness of handling and processing in addition to their excellent transparency and mechanical properties. However, in view of poor barrier properties against gaseous substance, such as oxygen, the packaged contents are easily spoiled by oxidation. Further, in view of easy transmission of flavor and carbon dioxide, the shelf life of the contents is unfavorably short.
A plastic container (bottle, etc.) mainly made of polyester such as polyethylene terephthalate has been widely used for storing tea, fruit juice, carbonated drink, etc. In the plastic bottles, small-sized plastic bottles have come to be dominantly used with years. Since the surface area per unit volume increases with decreasing size of the bottle, the shelf life of the contents packaged in a small-sized bottle is likely to become short. Recently, the application of the plastic container has been broadened, for example, by the sale of oxygen- and light-sensitive beer packaged in plastic bottle and the sale of hot tea packaged in plastic bottle. These new applications of plastic bottle require to still further improve the gas barrier properties.
To improve the barrier properties against gaseous substance, such as oxygen, it has been proposed to use film made of a combination of the thermoplastic resin mentioned above and a gas barrier resin, such as polyvinylidene chloride, ethylene-vinyl alcohol copolymer, and polyvinyl alcohol. Although a film laminated with polyvinylidene chloride shows good gas barrier properties independent of storage conditions, the combustion thereof produces dioxin, likely to cause environmental pollution. Ethylene-vinyl alcohol copolymer and polyvinyl alcohol are free from the environmental pollution. However, the gas barrier properties of a multi-layered film having a barrier layer made of these resins largely decrease when the contents with high water activity are stored, when the package is stored in atmosphere with high humidity, or when the package together with its contents are heat-treated for sterilization, although exhibiting good gas barrier properties in atmosphere with relatively low humidity.
A xylylene-containing polyamide obtained by the polycondensation of xylylenediamine and aliphatic dicarboxylic acid, particularly, polyamide MXD6 obtained from m-xylylenediamine and adipic acid has high barrier properties against gaseous substance, such as oxygen and carbon dioxide. As compared with ethylene-vinyl alcohol copolymer and polyvinyl alcohol mentioned above, polyamide MXD6 is superior in the gas barrier properties at high humidity, but slightly inferior in the gas barrier properties at moderate or low humidity.
A resin having function of capturing oxygen in a container while preventing outside oxygen from entering into the container has been developed and applied to a multi-layered bottle. In view of oxygen absorption speed, transparency, strength, and processability, a multi-layered bottle having a gas barrier layer made of polyamide MXD6 which is blended with a transition metal catalyst has been proposed as the oxygen-capturing bottle. However, the blended transition metal promotes the oxidative decomposition of polyamide MXD6 to reduce the strength of resin.
As an oxygen-capturing technique without using transition metal, an oxygen absorption method has been proposed (Patent Documents 1 and 2), which utilizes the oxidation of a reduced product of a reducible organic compound by oxygen. According to Patent Documents, the oxygen barrier properties are surely improved at room temperature and at low humidity. However, the proposed method requires an additional process of reducing the reducible organic compound, and therefore, the reduction in costs is desired.    Patent Document 1: JP Patent 2922306    Patent Document 2: JP 2001-514131T